1. Field of the Invention
The present invention relates to a fixing device and an image forming apparatus incorporating the same, and more particularly, to a fixing device that fixes a toner image in place on a recording medium with heat and pressure, and an electrophotographic image forming apparatus, such as a photocopier, facsimile machine, printer, plotter, or multifunctional machine incorporating several of those imaging functions, incorporating such a fixing device.
2. Discussion of the Background
In electrophotographic image forming apparatus, such as photocopiers, facsimiles, printers, plotters, or multifunctional machines incorporating several of those imaging functions, an image is formed by attracting toner particles to a photoconductive surface for subsequent transfer to a recording medium such as a sheet of paper. After transfer, the imaging process is followed by a fixing process using a fixing device, which permanently fixes the toner image in place on the recording medium by melting and settling the toner with heat and pressure.
Various types of fixing devices are known in the art, most of which employ a pair of generally cylindrical, looped belts or rollers, one being heated for fusing toner (“fuser member”) and the other being pressed against the heated one (“pressure member”), which together form a heated area of contact called a fixing nip through which a recording medium is passed to fix a toner image under heat and pressure.
One conventional type of fuser assembly employed in the fixing device is an endless belt looped for rotation around a hollow, generally cylindrical, stationary metal roller or pipe, typically formed by bending a thin sheet of conductive metal into a rolled configuration, which has its outer circumference entirely or partially facing the inner surface of the looped fuser belt. The metal roller is provided with a heater inside the hollow interior to conduct heat to its circumference, from which heat is radially transferred to the length of the fuser belt rotating around the metal roller.
Using a thin-walled metal roller allows for heating the fuser belt swiftly and uniformly, resulting in shorter periods of warm-up time and first-print time required to complete an initial print job upon startup, and high immunity against printing failures caused by insufficient heating of the fixing nip in high-speed application.
In specific configuration, the metal roller may have a fuser pad disposed outward from the roller interior and inward from the loop of the fuser belt for facing the inner surface of the fuser belt at the fixing nip. The fuser pad is used in combination with a flat, elongated reinforcing member accommodated within the metal roller to support the fuser pad under nip pressure. The reinforcing member extends across the generally cylindrical cross-section of the metal roller to partition the roller interior into narrow compartments, in one of which the roller heater is disposed facing the inner circumference of the roller.
One problem associated with such high-speed fuser assembly is that the fuser belt has its non-operating area (i.e., an area or extent that neither overlaps nor contacts a recording sheet passing through the fixing nip) excessively heated, and occasionally even thermally damaged, where the fixing device processes a number of recording sheets smaller than the width of the fuser belt in succession. Excessive heating of the fuser belt is attributed primarily to the use of a thin-walled metal roller in the fixing device. Having a low heat capacity and hence a fast thermal response, the metal roller immediately conducts heat for transfer to the fuser belt, which then accumulates heat along its sides where it does not contact the recording sheets being processed.
The problem is particularly pronounced where the fuser assembly is provided with a reinforcing member disposed inside the metal roller. This is because the heater, accommodated within a narrow compartment created by the reinforcing member partitioning the roller interior, is positioned in close proximity with the inner surface of the metal roller. Such positioning causes the heater to intensively heat the closest surface of the metal roller, resulting in a greater risk of excessively heating the non-operating area of the fuser belt than that with the roller interior unpartitioned.